The present invention relates to a brushless motor having an armature coil unit fixed to a casing, and a rotor magnet formed with an even number of poles and integrated with an output shaft for rotatably supporting the rotor magnet.
There has been known the conventional brushless motor of the spindle type used in office automation equipment, automatic machinery, medical instruments and so
FIG. 12 is a structural diagram of a prior art brushless motor. In manner similar to other types of motors, there are included a rotor magnet 3 and a stator composed of an armature coil unit 5. The rotor magnet 3 solely contributes to generation of output torque. The motor further has an output shaft 6 for rotatably supporting the rotor magnet 3. On the other hand, the stator is constructed such that the armature 4 composed of an armature coil unit 5 is fixed to an inner wall of a cylindrical casing 1 fixed to the bearing housing 2, while the casing 1 functions as a return yoke of the rotor magnet 3 and the armature coil unit 5. An electric current is fed to the armature coil unit 5 via lead wires 8. A pair of washers 9 are disposed on outer faces of the respective bearings 2 in order to suppress axial movement of the rotor magnet 3 during the course of rotation thereof. Further, the motor has not a sensor portion.
The conventional brushless motor has a winding structure shown in FIGS. 13, 14 and 15 wherein the armature coil unit 5 has a plurality of coils terminating in terminals or taps p, q, r and s. When the coils of the armature coil unit 5 are laid out as shown in FIGS. 14-15, they have a diameter dimension La. The armature coil unit 5 of the brushless motor is connected electrically as shown in FIG. 16 to form the armature 4 as shown in FIG. 18. As understood from FIG. 18, this winding is formed such that the armature coil unit 5 of the armature 4 has a radial thickness defined by two layers of the windings. However, since the radial thickness is limited to twice much as a diameter of the coil wire, the conventional structure has the drawback that the coil wires cannot be wound thick freely thereby limiting the amount of copper in the coil unit.
Particularly in reducing the motor size, while an energy product of the magnet has been improved efficiently, a magnetic motive force of the coil of the armature has not been improved efficiently. Stated otherwise, in reducing the motor size, the magnetic loading has been improved while the electric loading has not been improved. The motor output torque cannot be optimally improved unless a design balance is ensured with respect to a ratio between the magnetic loading and the electric loading. In view of this, it is necessary to broaden optimally a space gap between the magnet and the casing so as to increase the amount of the copper in armature coil unit. In order to increase the amount of copper in the coil, it is necessary to increase the radial thickness of the cylindrical coil unit.
It might be advisable to form multiple stages of the cylindrical coil units. However, for example, in the case that respective stages of the coil units are connected in parallel with each other as shown in FIG. 17, there may be caused the drawback that an inductive voltage coefficient Ke cannot be raised adequately in the multiple-stage motor. There is a problem that the series connection is needed in order to increase the value of Ke in the prior art.